Microbial enhanced oil recovery (MEOR) involves the use of microorganisms growing in-situ in an oil-bearing formation to facilitate either the production of materials to aid oil recovery or implementing a mechanism for oil recovery. Typically, the MEOR process is deployed as a supplement to the process of waterflooding which involves injecting water into the oil-bearing formation via a wellbore and recovering oil from a production well. The water used for injection is usually extracted from the oil-bearing formation as part of the oil production process or from other water bearing formations. The MEOR process is applied to waterfloods as a form of enhanced oil recovery (EOR) where microbes are introduced into the injection water via a line on the low pressure side of a pump that pumps the injection water from a holding tank into the oil-bearing formation. The inoculation process is typically a one-event process for each well and involves injecting several hundred liters of microbial consortia in the line on the low pressure side of the water injection pump.
Nutrients are also introduced into the injection water. The nutrients are introduced to enable the microbial activity in the oil-bearing formation. The nutrients may be introduced in the injection water when the injection water is in the holding tank. Alternatively, the nutrients may be introduced into the injection water via a line on the low pressure side of the water injection pump.
In addition to nutrients and depending on the type of microbes deployed, it may be necessary to introduce oxygen to support the MEOR process as some microbes use oxygen for certain microbial processes. The introduction of oxygen into the injection water is typically done by natural aspiration, which involves continuous exposure of the injection water to the atmosphere when the injection water is in the holding tank. The continuous exposure of the injection water to the atmosphere causes oxygen to be absorbed in the injection water. The absorption process reaches equilibrium when the concentration of oxygen in water is about 8 to 11 parts per million. Another method of introducing air into the injection water is to compress air into a line on the low or high pressure side of the pump that pumps the injection water from the holding tank into the oil-bearing formation. The holding tanks are usually closed so that exposure of the water to oxygen is restricted to minimize precipitation and potential corrosion as discussed below.
The injection water, containing nutrients and dissolved oxygen, is pumped to the wellhead, down the wellbore and into the oil-bearing formation. The presence of oxygen in the injection water, however, can cause corrosion (rusting) of tanks, pumps, flowlines and wellbore tubulars, which are all normally made of carbon steel. This is particularly problematic if the injection water has high salinity (high salt content). Solutions to this problem include the use of chemical inhibitors to prevent the corrosion or replacement of carbon steel equipment with stainless steel, fiberglass, plastic or epoxy resin coated materials to avoid corrosion and corrosion related equipment failures caused by the dissolved oxygen in the injection water. Chemical inhibitors can interfere with the MEOR process by altering microbial activity and for these systems replacement of the carbon steel equipment is required adding cost and time for process deployment.
What is more, the addition of the oxygen to the injection water can cause precipitation and scaling problems depending on the composition of the injection water. Precipitation causes skin damage, which restricts the flow of fluid to and from the wellbore. Further, the introduction of oxygen to the injection water may cause generation of biomass in the surface equipment that can further restrict the water flow. Accordingly, while MEOR processes that require oxygen improves oil recovery rates from oil-bearing formations, drawbacks such as corrosion of equipment, precipitation and blockages due to biomass limit its successful field implementation.